The term ‘different pile warp threads’ generally means that the pile warp threads have one or more mutually different appearance-determining properties. These properties are for example thickness, shine, hairiness or colour of the pile yarn or a combination of two or more of these properties. However, in the most preferred applications, these will be pile warp threads which differ at least by their colour.
The number of different pile warp threads per warp thread system determines how much variation is possible with regard to the pile. In order to produce pile fabrics, for example, in which the pile has five different colours, five pile warp threads having mutually different colours have to be provided for each warp thread system. In order to then produce a predetermined five-colour pattern in the pile, the pile warp thread of the desired colour is allowed to form pile in each warp thread system, at each pile point determined by the pattern, while the other pile warp threads of the same pile warp thread system are invisibly bound in the ground fabric. To this end, the different pile warp threads in each weft insertion cycle have to be positioned correctly. This is usually achieved using a jacquard machine. In order to increase the variation in the pile, the number of pile warp threads in each warp thread system has to be increased.
However, the number of warp threads which can be provided per meter on a weaving loom is limited by the physical properties of the machine and a number of technical limitations associated with weaving. Thus, with certain weaving looms and for a well-defined selection of yarns (indicated by the yarn number), it is practically impossible to weave at more than 1000 warp thread systems per meter (in the weft direction), while each warp thread system contains eight pile warp threads (8 colour frames). Above a certain upper limit, it is therefore no longer possible to increase the number of pile warp threads per warp thread system for a certain weaving loom, as a result of which there is also an upper limit for the possible pile variations of the pile fabrics which are woven therewith, when retaining the same number of warp thread systems per meter. This is also connected to the means which are available in the weaving loom to produce these pile fabrics, such as the weaving reeds with their associated reed dents, and the heddles which control ground warp threads and pile warp threads and a corresponding number of which should therefore also be present within the available space.
Thus, with the following yarns, it is for example possible to weave at 1000 warp thread systems per meter and 8 different pile warp threads per warp thread system (reed 1000, 8 colours):                Pile warp yarns with a density, given in ‘meter per gram of yarn’ or metric number (Nm), in the order of magnitude of 24/3 Nm, i.e. 8 Nm final,        associated weft threads for the ground fabric with yarn numbers 10/1 Nm or 14/1 Nm, and        associated external weft threads around which pile forming takes place, with a yarn number in the order of magnitude of 20/3 Nm or approximately 6 Nm final.        
The expression Nm final is understood to mean the number of meters per gram of an optionally compound yarn: 24/3 Nm means that the yarn is composed of 3 yarn components, each of which has a yarn number of 24 meters per gram. The compound yarn number is then 8 Nm or 8 meters per gram.
In this case, it is feasible to provide up to approximately 12 pile rows per cm in the warp direction in the pile fabric.
In order to be able to increase the pile variation, the number of warp thread systems per meter may be decreased so that more pile warp threads per warp thread system can be provided. However, this results in larger intermediate spaces between the successive pile rows in the weft direction, which reduces the number of pile points per cm in the weft direction and thus also the pile density (number of pile points per m2). Furthermore, it also results in a less uniform pile distribution.